This invention relates to telephone systems and more particularly to such systems which provide authorized telephone operators access to busy subscriber circuits for verification purposes.
A verification network in a telephone system is designed to enable only an authorized telephone operator to connect through a toll switch to the telephone lines of a subscriber whose telephone is in use or appears to be in use. Such access is limited to special circumstances, such as an emergency, warranting the temporary compromise of subscriber privacy. Because of the obvious need to strictly limit such access to subscriber telephone lines to authorized telephone operators, each source of a verification call must be tested and passed upon as as a precondition to access. It is further essential that this test cannot be duplicated or simulated by unauthorized persons. This invention is directed to a network which performs this function.
A prior art technique for coping with this problem utilizes a single direction audio tone handshake signal generated by the operator and detected by a tone decoder which responds to the handshake signal for providing the desired access. Such a technique is used where a D.C. method of handshake would interfere with low impedance dial pulse and supervisory signals transmitted over transmission lines.